1. Field of the Invention
The present invention is directed to an optical measurement system for determination of an object profile, and more particularly to a system which scans an object surface by a light beam and analyzes the reflected light beam therefrom in order to determine the object profile based upon position data of scanned points on the object surface.
2. Description of the Prior Art
An optical measurement system for determination of an object profile based upon the position data on the object surface has been proposed, for example, to check mounting conditions of electronic components on a printed board or soldering condition thereof. Such optical measurement is disclosed in the U.S. patent application Ser. No. 463,579 filed on Jan. 11, 1990 by the same applicant. As shown in FIG. 7 of the attached drawings, the above prior art system utilizes a laser beam I which is emitted from a light source 1 and directed toward a surface of an object 9 through a projecting lens 3 to provide a beam spot on the object surface. The light beam I is deflected by a first vibration mirror 2a to move the beam spots across the object surface for scanning the object surface. A position detector 8 is disposed to monitors a reflected light beam R reflected from the object surface in a direction angled from the incident light beam I and directed through a receiving lens 4. A second vibration mirror 2b is positioned between the receiving lens 4 and the detector 8 and is vibrated in synchronism with the first mirror 2a to direct the reflected light beam R to the detector 8. During the scanning operation, the detector 8 acknowledges a change in an incidence of angle of the reflected light beam R and produces a position signal indicative of the varying angle of incidence of the reflected light beam R. The position signal is processed to measure instantaneous distances to individual scan points on the object surface by triangulation. Thus obtained measured distances are related to individual height dimensions of the scanned points and are analyzed to provide a profile of the object surface along the scanned points. Disposed between the light source 1 and the first vibration mirror 2a are a collimator 5 and mirrors 6 and 7.
With this system, it is known that, as shown in FIG. 8, two spaced points A and B at the same height level on the object surface are detected at the same point A' on the detector 8. However, if the first and second vibration mirrors 2a and 2b fail to synchronize even at a slight extent, the above two points A and B on the object surface are detected on the detector 8 not at the common point A' but respectively at points A' and B' which are offset along x-direction by a distance of .DELTA.x. Since the detector 8 acknowledges the displacement of the reflected beam spot in the z-direction as a change in an incidence of angle of the reflected light beam R for the triangulation, the above deviation .DELTA.x will not affect the result of the triangulation. Nevertheless, in order to compensate for the deviation .DELTA.x and keep detection reliability, the detector 8 is required to have its sensing surface widened in the x-direction enough to encompass the deviation .DELTA.x. Accordingly, when the detector is designed in the form of an array sensor, extra sensor arrays are required to be arranged to cover an increased width W in the x-direction with attendant increase in economy and complication of the system.
In the meanwhile, when checking the soldering condition by scanning the light beam across the surface of a solder 90, as shown in FIG. 9, a secondary reflected light beam R' is likely to occur due to lucid surface of the solder, in addition to the intended reflected beam R from the solder surface. Such secondary reflected light beam R' will mislead the detection at the detector 8 and therefore give false height data and should be prevented from entering the detector 8 for an accurate measurement purpose. However, when the detector 8 is required to have a widened sensor surface or more number of sensor arrays for the reason as above, the result is an increased probability of the secondary reflected light beam R' entering the detector to thereby lower the measurement accuracy.
In an attempt to avoid the above problems, it has been proposed in U.S. Pat. No. 4,627,734 to use a single deflector commonly for projecting the light beam toward the object surface as well as for receiving the reflected light beam therefrom. The deflector of this patent is a pyramidal or polygonal mirror having several facets or faces which are separately utilized to project the light beam toward the object surface and receive the reflected light beam therefrom. Therefore, there still remain like deviation of the detected points on the side of the detector due to possible variation in planar condition or accuracy between the plural facets or faces.
Another prior optical measurement system is disclosed in U.S. Pat. No. 3,866,038 which utilizes a polygonal mirror having plural faces for projecting light beam toward the object surface and receiving the reflected light therefrom on the common face. With this system, therefore, the above problem can be eliminated. However, there is another problem in this patent due to the configuration that a single lens is disposed between the deflector and the object surface for directing therethrough the light beam toward the object surface and the reflected light beam therefrom. In view of that the common lens is required to focus the light beam to the object surface and that the reflected light beam travels substantially the same distance from the object surface to the common lens as the light beam does from the common lens to the object surface, the reflected light beam from the object surface will be directed to the deflector as parallel light beams after passing through the common lens, which necessitates an extra convergent lens between the deflector and the detector in order to have the reflected beam focused on the sensing surface of the detector, which complicates the optical configuration of the system. Further, due to use of the single lens commonly for directing the deflected light beam to the object and directing the reflected light beam to the deflector, the light beam being deflected is restricted to pass through a peripheral region of the lens and therefore is susceptible to spherical aberration and is difficult to attain wide scan width on the object surface. In addition, the optical system of this patent will incur an undesirable interference between the light beam directed toward the object surface and the reflected beam therefrom since diffused lights produced on or within the lens as the light beam is directed through the lens to the object surface have more chance of intruding into a receiving light path of directing the reflected light beam toward the deflector, which lowers an S/N ratio with an attendant decrease in detection accuracy.